I used this recently to speed up some symbolic manipulation. I was using SymPy but its simplify is really quite slow and the documentation basically just says "work out exactly which simplifications you need and apply them", however in my case I needed something completely general. Now I check whether wolframscript is installed and then just call it in the command line. It's much faster for complicated simplifications, but also much slower for simple stuff (I guess it has a long start up time)

SymPy has a built-in converter to/from Mathematica (although it can't convert from Mathematica lists yet), so that's helpful

(Here's the project I used it in for interested people https://github.com/joebentley/simba)